The present invention relates to a making method for an optical disk such as a video disk or a compact disk, more particularly for a high-density recording optical disk.
In making an optical disk having an information recording portion formed from the presence and absence of recesses or pits, the optical disk is generally obtained by preparing a stamper having a recessed surface according to recording information and molding resin with use of the stamper.
Such a conventional making method for an optical disk using the stamper will now be described with reference to FIGS. 10A to 10D showing the steps of the making method.
Referring to FIG. 10A, a photoresist layer 2 is entirely applied to a smooth surface la of a substrate 1 such as a glass substrate, and the photoresist layer 2 is sequentially exposed by scanning a laser beam modulated according to information to be recorded, e.g., signals of "0" and "1". Subsequently, development is carried out to form a plurality of apertures 3 arranged in a desired pattern according to the recording information.
Then, as shown in FIG. 10B, a conductor layer such as Ni is entirely formed on the photoresist layer 2 including the apertures 3 by electroless plating, vapor deposition, sputtering, etc., and a thick metal layer 4 such as Ni is then formed on the conductor layer by electroplating.
As shown in FIG. 10C, the metal layer 4 is separated from the glass substrate 1 to prepare a stamper 5.
As shown in FIG. 10D, resin such as acrylic resin or polycarbonate resin is molded with use of the stamper 5 to obtain an optical disk 7 having a plurality of recesses 6 as information pits arranged in a reverse pattern as transferred from a recessed surface of the stamper 5.
However, each recess or pit 6 of the optical disk 7 thus obtained has a sectionally trapezoidal shape such that opposite side surfaces of each recess 6 are inclined so as to follow the shape of each aperture 3 of the photoresist layer 2. Such inclined side surfaces of each recess 6 causes a problem in reproduction characteristics.
Such inclination of the side surfaces of each recess 6 is generated as follows:
The formation of each aperture 3 through the photoresist layer 2 by pattern exposure of a laser beam will be described with reference to FIGS. 11A and 11B.
In general, when a laser beam h.nu. is condensed and irradiated to the photoresist layer 2 on the glass substrate 1, the laser beam h.nu. is distributed as shown in FIG. 11A. In this case, an intensity distribution of the laser beam h.nu. is shown by a one-dot chain line a in FIG. 11A. Further, an area to be exposed to enable the development under the above condition is an area surrounded by a dashed line b in FIG. 11A. This area has a shape according to the intensity distribution of the laser beam h.nu..
Accordingly, when the photoresist layer 2 is developed after the exposure, the exposed area mentioned above is made into the aperture 3, which has a sectionally trapezoidal shape such that a width W.sub.1 on the side of incidence of the laser beam is larger than a width W.sub.2 on the side of the glass substrate 1, that is, at the boundary between the aperture 3 and the glass substrate 1.
To solve the problem due to the sectionally trapezoidal shape of each pit in making the optical disk, the present applicant has proposed an improvement in the making method for the high-density optical disk in Japanese Patent Application No. 1-244463.
FIGS. 12A to 12E show the steps of the making method for the optical disk proposed in the above cited reference. Referring to FIG. 12A, a substrate 1 such as a glass substrate having a surface layer 8 such as Cr is first provided. The surface layer 8 has a thickness t corresponding to a depth of each information pit (recess) to be formed on an intended optical disk, and shows an etching property different from that of the substrate 1 The surface layer 8 is formed by vapor deposition, sputtering, etc. Then, as previously mentioned, a photoresist layer 2 is formed on the surface layer 8, and is exposed and developed to form a plurality of apertures 3. As apparent from FIG. 12A, each aperture 3 has a sectionally trapezoidal shape such that a width W.sub.1 of each aperture 3 on the open side thereof is larger than a width W.sub.2 of each aperture 3 on the bottom side thereof, as similar to the illustration in FIGS. 11A and 11B.
Then, as shown in FIG. 12B, the surface layer 8 is etched over its whole thickness by perpendicular anisotropic etching such as RIE (reactive ion etching) with the photoresist layer 2 used as a mask. By applying the RIE, the surface layer 8 only is selectively etched, and the substrate 1 itself is not etched or hardly etched. According to this method, the surface layer 8 is etched through the apertures 3 to form a plurality of recesses 9 each having a width corresponding to the small width W.sub.2 of each aperture 3 of the photoresist layer 2 and having opposite substantially vertical side surfaces.
Then, as shown in FIG. 12C, after removing the photoresist layer 2, a metal layer 4 such as Ni is entirely formed on the surface layer 8 including the recesses 9, as similar to the illustration in FIG. 10B. Thereafter, the metal layer 4 is separated from the substrate 1 to obtain a stamper 5 having a desired recessed surface according to recording information.
The stamper 5 thus obtained has a plurality of projections each having opposite vertical side surfaces. Accordingly, as shown in FIG. 12E, an optical disk 7 molded by using the stamper 5 is formed with a plurality of pits or recesses 6 each having opposite vertical side surfaces and a width corresponding to the small width W.sub.2.
In the optical disk 7 formed in this manner, the pits (recesses) 6 can be made very small to thereby realize high-density recording.
In the making method mentioned above, a light intensity distribution of the laser beam modulated according to recording information is symmetrical with respect to an optical axis, that is, the intensity distribution illustrated in FIG. 11A is uniform in any plane containing the optical axis. In other words, each pit (recess) of the optical disk has the same width corresponding to the width W.sub.2 in respect of both a direction of relative movement of a reproducing optical head of an optical disk drive, i.e., a direction along a recording track of the optical disk (which direction will be hereinafter referred to as a circumferential direction or .theta. direction) and a direction perpendicular to the circumferential direction (which direction will be hereinafter referred to as a radial direction or r direction).
The apertures 3 of the photoresist layer 2 each aperture having the relation of W.sub.1 &gt;W.sub.2 in accordance with the above method are arranged with a minimum spacing in relation to the function of the etching mask and other conditions, thereby reducing the pitch to realize high-density recording. However, as shown in FIG. 12E, when a width P corresponding to one signal is minimized, that is, when the sum P of a width W.sub.P of a recess 6 corresponding to a recording portion for information "1" and a width W.sub.L of a land formed adjacent to the recess 6 is minimized, the width W.sub.P becomes smaller than the width W.sub.L. Also in respect of the scanning direction of a reading light on the optical disk (i.e., the .theta. direction), the relation of W.sub.P &lt;W.sub.L holds, so that a ratio between the width W.sub.P and the width W.sub.L in respect of the .theta. direction does not become 1:1, that is, a duty ratio does not become 50%. Accordingly, when the reading light is irradiated onto the optical disk to read the pits or Information, a duty ratio of electrical signals corresponding to the information "0" and "1" does not become 50%, causing a reduction in S/N (C/N).